To test your alias you can load your assembly via the #reference directive or if you packaged it via the #addin directive.
You can then call your alias in a script using HelloWorld() the ICakeContext can be omitted as it's automatically provided by the Cake core engine.

Example "HelloWorld" Cake script:

#reference "HelloWorld.dll"
HelloWorld();

If you have any namespaces you want to automatically import for the user, then you do that using the CakeNamespaceImport attribute to your extension method, example: